1. Field of the Invention
The invention relates to a manhole cover that is particularly simple and inexpensive to manufacture.
2. Discussion of the Prior Art
GB 2 145 444 A, the closest prior art, discloses a manhole cover that comprises only the two components of a base and a filling. The preferred material for the base of the manhole cover is the same material that is used for a manhole that is to be covered by the manhole cover, whereby, high-density polyethylene is named as an example. The manhole serves as access to the sewage system for inspection purposes. The disclosure does not mention how the manhole cover is arranged and what loads it can withstand.
An unconventional manhole cover is disclosed in DE 298 01 640 U1. This manhole cover has a metal outer shell on its bottom side, into which the actual base, which has a certain layer thickness, is then cast, whereby the filling is then cast into the base. The production of this manhole cover, which overall comprises a three-layer construction, is therefore technically complicated. The components or materials used are relatively expensive. Thus, for example, the outer shell is provided as a break-mold made of metal, and the base is constructed either of cast steel or of polymer concrete, the polymer concrete having thicker dimensions. The filling is also made of polymer concrete. The manhole cover is to be arranged in a track system and is constructed to withstand foreseeable high stresses caused by a high volume of traffic, high mechanical stresses, and aggressive climatic conditions.
U.S. Pat. No. 5,123,776 discloses a manhole cover that is constructed as a filled-in hollow body. A sheath surrounds the filling on all sides, so that the filling must be introduced into the sheath through a filler opening. The sheath is preferably made of polyethylene, and the filling preferably of concrete. Handles for lifting purposes are basically constructed as U-shaped brackets that open downward. They can be pulled up out of the manhole cover in a vertical direction, so that they can be gripped in order to raise the manhole cover. When in their lowered non-operational state, the lifting handles each rest in a recess that is provided in the surface of the manhole cover. The production of the manhole cover is complicated, because a hollow body must be filled, whereby providing the openings in the hollow body necessary for the vertical mobility of the lifting handles complicates the filling process. or, if the openings are not formed in the filling process, after the filling has hardened, several through-bores have to be made in the manhole cover to create channels through which the lifting handles can extend.
U.S. Pat. No. 4,974,992 discloses a cover that is not used as a manhole cover, but rather, is constructed as a metal plate and serves to cover trenches in street surfaces. The cover is used when digging work is suspended and traffic is allowed on the street surface. In order to withstand the traffic loads, the metal plates have appropriately sturdy dimensions and are consequently heavy. In order to simplify their handling, Omega-shaped handles that are rotatable on both ends are provided in the cover plate and can be swiveled from their downward-hanging resting position into an upright operational position in which they project upwards above the surface of the cover.
U.S. Pat. No. 4,801,483 discloses a cover that has a core with a cell structure, as well as an external, closed sheath layer made of a fiber-reinforced resin. The cover is to specifically sustain high dynamic loads, so that, e.g., the vehicles in road traffic can drive over the cover. The core can, e.g., be constructed of multiple balsa wood blocks, and the sheath layer of fiberglass-reinforced epoxy or polyester resin. The manufacture of the cover is complicated, because two separate parts of the sheath layer are produced. A first part of the sheath layer forms a recess into which the individual components of the core are inserted. The core is then covered with the second part of the sheath layer. One must ensure that both parts of the sheath layer are connected to each other at all contact points they share with one another and with the core structure, and that all air bubbles be avoided during this process, in order to create an essentially monolithic mass. Aids for lifting the cover are not provided; rather, the cover has armature bores that allow ground anchors to be inserted, so that the cover can be affixed to the substrate with the aid of these ground anchors.
In numerous instances, manhole covers are necessary in places other than streets. For example, inspection and maintenance manholes may be provided on private properties, e.g., in garage entrances, in order to provide access to the building's sewage system installations for cleaning and inspection purposes. Mechanical stress caused by heavy trucks is unlikely here, as is heavy traffic or high exposure to de-icing salt in the winter. Conventional manhole covers are therefore frequently overqualified for these applications and, due to their complicated production process or the costly materials used. are unnecessarily expensive.
The object of the invention is to improve a generic manhole cover in such a way that it can be produced as inexpensively as possible.